As well known, a general Modular Multilevel Converter (MMC) consists of one or more phase modules 1 as illustrated in FIG. 1, and each of the phase modules 1 has N sub-modules 10 connected in series with each other, each of the N sub-modules having two output terminals X1 and X2. Also, load connection terminals L1, L2 and L3 may be connected to 3-phase loads, for example, a 3-phase AC power system. The phase module 1 is divided into an upper part phase module 1a and a lower part phase module 1b based on the load connection terminals L1, L2 and L3.
Generally, an MMC uses a Pulse Width Modulation (PWM) method to switch sub-modules. Among various PWM methods, a Phase-shifted Carrier (PSC)-PWM method is advantageous in that multilevel output may be produced and in that fewer harmonic waves are output even at a low switching frequency.
FIG. 2 is a view illustrating the carriers of PSC-PWM applied to a conventional MMC.
Referring to FIG. 2, in the conventional MMC, when a desired reference signal 21 has a value that ranges from −1 to 1, the carrier signals 22, assigned to the N sub-modules 10, are configured to oscillate within a range between −1 and 1. Here, the reference signal 21 is compared with the carrier signals 22 assigned to the N sub-modules 10, and the sub-module 10, the triangular wave 22 of the carrier signal of which is higher than the reference signal 21, is switched on, but the sub-module 10, the triangular wave 22 of the carrier signal of which is lower than the reference signal, is switched off. The on/off switching of the sub-modules is controlled by a controller (not illustrated).
In the conventional method, N carriers must be retained. In other words, if any one of the N sub-modules fails, the N carriers are not retained, thus adversely affecting the operation of the MMC.
In order to solve the above problem, in the conventional art a method was proposed in which M redundant sub-modules are additionally arranged in addition to the N sub-modules that participate in the operation, and the operation is performed using the N+M sub-modules. However, in this case, the carriers must continuously change N sub-modules, among the M+N sub-modules. For example, if the carriers of the first to N-th sub-modules are maintained in the current cycle, the carriers must be changed in the next cycle such that the carriers of the second to (N+1)-th sub-modules are maintained. Here, when the carriers are changed, because the switching states are also changed, which incurs unnecessary switching, the switching frequency is increased. Also, it is difficult to implement a controller capable of rotating the carriers.